FIELD OF THE INVENTION
The present invention relates to a process for preparing highly concentrated aqueous solutions of low molecular mass homopolymers of N-vinylpyrrolidone.
The preparation of N-vinylpyrrolidone polymers by free-radical polymerization is known. The mechanism of polymerization under various conditions is described, for example, in Polymer Journal, 17 (1985) 143-152. Polymerization in organic solvents, for example in alcoholic solution in accordance with U.S. Pat. No. 4,053,696, leads to polyvinylpyrrolidone of low molecular mass, since the organic solvents may act as chain regulators. To prepare highly concentrated aqueous solutions of the polymer, however, at least the majority of the organic solvents have to be distilled off and then either disposed of or reprocessed.
The polymerization of N-vinylpyrrolidone in aqueous solution has to date usually been carried out in the presence of hydrogen peroxide as initiator, as is described, for example, in U.S. Pat. No. 2,335,454. In this case the molecular weight of the polyvinylpyrrolidone depends on the hydrogen peroxide concentration: low molecular weights result from high hydrogen peroxide concentrations, and vice versa. Highly concentrated aqueous solutions of the polyvinylpyrrolidone, however, cannot be prepared, since high monomer concentrations of &gt;30% by weight make it impossible to control the exothermic reaction, and in the case of feed techniques the highly grafting effect of the hydrogen peroxide leads to an unwanted buildup of molecular weight.
WO 94/26796 discloses the preparation of low molecular mass copolymers of N-vinylimidazole and N-vinylpyrrolidone using an azo initiator in aqueous solution in the presence of a sulfur compound as regulator.
DE 2 218 935 describes the polymerization of N-vinylpyrrolidone, alone or in a mixture with minor amounts of other monoolefinically unsaturated monomers, in an aqueous medium in the presence of water-insoluble polymerization catalysts which form free radicals and which are added in the form of a fine suspension in an aqueous solution of the N-vinylpyrrolidone polymers.
It is an object of the present invention to provide a simple process for preparing highly concentrated aqueous solutions of low molecular mass homopolymers of N-vinylpyrrolidone.
We have found that this object is achieved by carrying out polymerization in aqueous solution in the presence of particular polymerization regulators.